Pumps intended for pumping contaminated liquid comprising solid matter generally comprise a so-called upper gap that divides the pump chamber of the pump into a first part where the impeller of the pump is situated, and a second part where the drive shaft sealing assembly of the pump is adjoining. The primary object of the gap is to prevent impurities that are found in the pumped liquid from coming into contact with and thereby risk destroying the drive shaft sealing assembly.
Agricultural waste consists of water, urine, cattle manure, straw bedding, etc., that is, the solid matter consists of long-fibred, cellulose-containing constituents such as straw from the bed of the cattle, non-decomposed organic material in the form of stems and other hay constituents from the cattle manure, etc. It should be pointed out that other applications that are intended to pump solid matter, and then in particular solid matter with long-fibred, cellulose-containing constituents, are impaired by similar problems, which hereinbelow will be described exemplified in the context of the pumping of agricultural waste, without because of this being limited thereby.
Contaminated liquid comprising agricultural waste and the like is a disadvantageous liquid to pump, because of the solid matter attaching to, among other things, the surfaces that define the above-mentioned gap, which leads to clogging of the gap and the risk of solid matter filling and cramming the space between the upper cover plate of the impeller and the seal housing cover. These problems result in imbalance in the rotating unit, detrimental lateral forces and retarding frictional forces, which leads to the risk of breakage of the pump and above all the drive unit of the pump.
In applications that are utmost cumbersome, there is in practice only one solution to solve the problem of clogging, namely to increase the width of said gap to become 2-3 times wider than usual, that is, the width of the gap is increased to several millimetres. However, it has turned out that an increase of the width of the gap entails that the drive shaft sealing assembly has a greater tendency to be damaged, and if the drive shaft sealing assembly is damaged, pumped liquid leaks into the sealing unit and the drive unit, and this in turn leads to total loss of the pump.
Today, there are also other attempts to prevent the so-called upper gap from clogging, see, for instance, U.S. Pat. No. 4,472,109 and SE 391 008. However, the device shown in these documents is explicitly arranged in such a way that the surfaces that mutually delimit the upper gap should contain recesses at the same time as the recesses must not extend axially along the entire upper gap, i.e., that the two gap-delimiting surfaces should have rotationally symmetrical surfaces with the purpose of keeping the cross-sectional area of the gap at a minimum. Furthermore, the embodiment examples shown should have recesses with axially extending edges on the inner as well as the outer gap-delimiting surface, which entails an imminent risk of said edges hooking up in each other and damaging the pump.